1. Field of the Invention
This invention relates to an improved anodized aluminum susceptor for use in apparatus for forming integrated circuit structures, and a method of making such a susceptor. More particularly, this invention relates to an anodized susceptor, and method of making the same, wherein the anodized coating on the susceptor is capable of withstanding high temperatures and NF.sub.3 plasma without peeling and without contaminating a wafer processed thereon with impurities contained in the aluminum susceptor.
2. Description of the Related Art
In the formation of integrated circuit structures, certain processes such as plasma-assisted chemical vapor deposition processes are carried out in a vacuum chamber of a reactor wherein the wafer is mounted on a platform referred to as a susceptor which usually serves not only as a support for the wafer, but also as one of the electrodes for generation of the plasma. The susceptor is usually formed of aluminum, due to the availability and cost of the metal, as well as its machinability.
It is customary to anodize the aluminum susceptor, to protect the susceptor surface, and to provide an aluminum oxide surface which is relatively inactive to the processing being carried out in the reactor during deposition on the wafer. For example, the anodized surface prevents or inhibits reaction of the aluminum surface with fluorine-containing processing gases such as NF.sub.3, SF.sub.6, or fluorocarbons, which may form aluminum fluoride on the susceptor surface.
Conventionally, such aluminum susceptors have been formed from AA6061 aluminum alloy because of this alloy's additional mechanical strength at room temperature, even though it is known that the mechanical strength of this alloy at the operating temperatures used in processing semiconductor wafers is not necessarily higher than other aluminum alloys.
Formation of an anodized coating on such aluminum susceptors made from AA6061 aluminum alloy, using conventional sulfuric acid anodizing, provides an anodized layer of aluminum oxide capable of withstanding the corrosive reaction of fluorine-based chemistry up to temperatures high as 475.degree. C. in the presence of a plasma. However, this conventional aluminum susceptor does experience cracking of the anodized film.
Also, the AA aluminum alloy 6061 contains a number of alloying elements, added thereto to achieve certain properties, including from 0.8 to 1.2 wt. % magnesium. It has been found that under certain conditions encountered when processing a wafer mounted on such an anodized susceptor, magnesium atoms in the AA 6061 alloy migrate through an anodized aluminum oxide coating and detrimentally interfere with the deposition being carried out in the chamber.
When aluminum alloys having a lower magnesium content were substituted for the AA6061 alloy, e.g., AA1100 alloy, in an attempt to solve the magnesium contamination problem, it was discovered that such higher purity alloys did not form an anodic coating in sulfuric acid which would withstand cracking as well as peeling when exposed to temperatures as high as 590.degree. C., or a combination of an NF.sub.3 plasma and an elevated temperature of 475.degree. C.
While surface roughening of the surface of an aluminum alloy, prior to anodization, has been previously practiced for other purposes and such surface roughening does overcome the cracking and peeling problem, it has also been found the use of sulfuric acid itself as the anodization electrolyte can be a source of contamination. Anodization of aluminum alloys in a sulfuric acid electrolyte have been found to result in the formation of particles having an undesirably high sulfur content.
U.S. Pat. No. 5,039,388 discloses forming an anodic coating on a high purity aluminum electrode using a chromic acid electrolyte instead of sulfuric acid, as noted. However, such coating can still suffer from high decomposition and resultant coating loss, particularly at higher temperature applications, e.g., 590.degree. C., and the resultant contamination problems attendant therewith. Furthermore, the use of anodizing electrolytes containing chromium ions can result in the formation of an anodized layer containing chromium, a substance known to reduce device reliability and stability, particularly in CVD-formed dielectric coatings, apparently because of the mobility of the chromium.
Thus, there is a great need for forming an anodized coating on an electrode formed from an aluminum alloy, such as a high purity aluminum alloy, which will withstand these conditions and prevent contamination of the wafer. The present invention provides such a anodized coating on an electrode formed from an aluminum alloy, particularly a high purity aluminum alloy electrode.